1. Field of the Invention
The present invention relates to a laser processing method for forming a plurality of grooves in relation to a thin platelike workpiece such as a semiconductor wafer by applying a laser beam, and more particularly to a laser processing method suitable in the case that the grooves intersect each other.
2. Description of the Related Art
In a fabrication process for semiconductor devices (chips), a plurality of rectangular chip areas are partitioned by a plurality of crossing streets (division lines) formed on the front side of a substantially disk-shaped semiconductor wafer, and an electronic circuit such as IC or LSI is formed in each chip area. Thereafter, the back side of the wafer is ground as required and the wafer is next cut to be divided along all of the streets (dicing step) to obtain the individual chip areas as semiconductor chips. Each semiconductor chip thus obtained is packaged with a sealing resin and is widely used for various electric or electronic equipment such as a mobile phone or PC (personal computer).
As the dicing step for dividing the wafer into the individual semiconductor chips, blade dicing is generally known in the art, wherein a thin disk-shaped blade rotating at a high speed is cut into the wafer. It is known that the blade dicing has an advantage such that a flat and sharp cut surface can be obtained.
In recent years, laser dicing has also been adopted, wherein a transmissive laser beam is applied along each street to cut and divide the wafer. For example, Japanese Patent Laid-open No. Hei 10-305420 has proposed a technique of first applying a laser beam along each street on the front side of a wafer to form a plurality of grooves and next breaking (cleaving) the wafer along these grooves to obtain individual chips. The formation of the grooves along the streets of the wafer is effected by a thermal evaporation phenomenon called ablation such that the component of the wafer is heated and evaporated by a laser beam.
In performing the laser dicing, a plurality of grooves (first grooves) are first formed along all of the streets (first streets) extending in one direction as described in the above publication, and a plurality of second grooves are next formed along the second streets perpendicular to the first streets. Accordingly, the laser beam applied along the second streets is moved at right angles to the first grooves formed along the first streets. There is a case that a portion of each chip near the intersection of each first street and each second street is influenced by overheating at the front end of each second groove immediately before reaching each first groove.
The influence by overheating includes a deformation of each chip 3 shown in FIG. 7, wherein the width of the second groove G2 at a portion immediately before the first groove G1 extends so that the corners of the adjacent chips 3 on the opposite sides of the second groove G2 are not square, but round as shown in FIG. 7. The arrow shown in FIG. 7 denotes the direction of movement of the laser beam applied along each second street. Further, the round corners of each chip 3 become weak to cause a reduction in die strength, resulting in cracking or breakage in some case. Further, the influence by overheating also includes a reduction in easiness of a pickup operation such that each chip 3 is picked up after ending the dicing step.
The easiness of the pickup operation will now be described. In general, the wafer itself is hard to handle in the dicing step, and the wafer is therefore attached to a dicing tape to which a dicing frame is attached, thereby making the wafer handling easy. After ending the dicing step, each chip is peeled off from the dicing tape and picked up. In this case, the higher the peelability of each chip, the more easily each chip can be picked up. In this respect, a UV (ultraviolet) curing type adhesive material is used as the adhesive layer formed on the dicing tape, and ultraviolet light is applied to the dicing tape in picking up each chip, thereby reducing the adhesive strength of the adhesive material. As a result, each chip can be easily picked up.
However, in applying a laser beam along each second street after forming each first groove along each first street in the conventional dicing step, the portion of each second street immediately before reaching each first groove is overheated by the laser beam as mentioned above. Accordingly, the UV curing type adhesive material of the dicing tape at this portion is deteriorated by the overheating, causing a problem such that the adhesive strength of the adhesive material is not reduced in spite of the application of ultraviolet light. As a result, each chip is hard to peel off at this portion from the dicing tape in the pickup operation, thus reducing the easiness of the pickup operation. This problem occurs both in the formation of a groove having a depth smaller than the thickness of the wafer and in the formation of a slit cut through the thickness of the wafer. The deformation of each chip and a reduction in die strength of each chip as mentioned above occur more remarkably in the case of full cutting than in the case of groove formation.